Monoclonal antibodies have been obtained in the past year by hybridoma techniques against the rabbit T cell tumor line RL-5 and against mouse histocompatibility antigen H-2.31. Antibodies to H-2.31 are being propagated to determine whether they can be usefully employed in structural studies. The range of antigens under investigation has been broadened to include simple proteins, such as ovalbumin and insulin. Monoclonal antibodies to these proteins have been prepared for idiotypy studies.